A number of years ago it was discovered that steel pipelines transmitting gas under substantial pressure in contact with or buried in the soil were subject to stress-corrosion cracking and consequent failure. Since that time considerable work has been done in an effort to determine the causes for such stress-corrosion cracking and to find solutions to the problem. In general, such studies have concentrated on three areas, i.e., (1) modifying the steel from which the pipes are made, (2) modifying operating practices involving stress, temperature and cathodic protection, and (3) modifying the environment. It presently appears that the most promising approach to the protection of pipelines against such stress-corrosion cracking resides in modification of the environment of the pipeline. This may be done in a variety of ways including incorporation of an appropriate inhibitor in the primer or other coating on the pipeline or any other means for introducing an effective inhibitor to the environment adjacent the pipeline.
Considerable work has been done to identify the causes of stress-corrosion cracking and in particular those environments which may encourage such cracking. Field and laboratory studies have indicated that the most probable environments that may cause stress-corrosion cracking are aqueous solutions of bicarbonate, carbonate-bicarbonate mixtures, hydroxides or nitrates. More specifically, chemical analyses of liquids found under coatings near the locations of stress-corrosion cracks in buried pipelines have shown that the principal components in most of the liquid samples were carbonate and bicarbonate ions. Some of the samples tested also contained small amounts of other ions such as the hydroxide ion, chloride ion and nitrate ion. In addition, crystals of sodium bicarbonate have been found on pipe surfaces near stress-corrosion cracks. A review of the literature indicates that nitrates and hydroxides can cause stress-corrosion cracking of mild steels but until recently there had been no evidence that carbonates or bicarbonates could cause such cracking. However, laboratory studies have indicated that stress-corrosion cracks can be produced in pipe steel using sodium carbonate-bicarbonate solutions and sodium bicarbonate solutions. Therefore, it is now believed that stress-corrosion cracking of buried pipelines is encouraged by carbonate-, hydroxide- and nitrate-containing environments.